Centrifugal pump for conveying a fluid

ABSTRACT

A centrifugal pump for conveying a fluid includes a pump housing, an impeller to convey the fluid from an inlet to an outlet, a shaft to rotate the impeller, a first sealing device to seal the shaft at a suction side, a second sealing device to seal the shaft at a discharge side, a balance drum connected to the shaft between the impeller and the second sealing device, the balance drum defining a front side facing the at least one impeller and a back side facing the second sealing device, a relief passage between the balance drum and a stationary part, a balance line connecting the back side with the suction side, a discharge opening arranged at the relief passage between the front side and the back side, and a connecting line connecting the discharge opening with the first sealing device.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to European Patent Application No.20169858.6, filed Apr. 16, 2020, the contents of which are herebyincorporated herein by reference in their entirety.

BACKGROUND Field of the Invention

The invention relates to a centrifugal pump for conveying a fluid.

Background Information

Conventional centrifugal pumps for conveying a fluid, for example aliquid such as water, are used in many different industries. Someexamples are the oil and gas industry, the power generation industry,the chemical industry, the water industry or the pulp and paperindustry. Centrifugal pumps generally have at least one impeller and ashaft for rotating the impeller. The at least one impeller can beconfigured for example as a radial impeller or as an axial or semi-axialimpeller or as a helicoaxial impeller. Furthermore, the impeller can beconfigured as an open impeller or as a closed impeller, where a shroudis provided on the impeller, the shroud at least partially covering thevanes of the impeller.

A centrifugal pump can be designed as a single stage pump having onlyone impeller mounted to the shaft or as a multistage pump comprising aplurality of impellers, wherein the impellers are arranged in series onthe shaft.

Many centrifugal pumps include at least one balancing device for atleast partially balancing the axial thrust that is generated by theimpeller(s) during operation of the pump. The balancing device reducesthe axial thrust that is acting on the axial bearing or the thrustbearing. The balancing device can comprise a balance drum for at leastpartially balancing the axial thrust that is generated by the rotatingimpellers. The balance drum is fixedly connected to the shaft of thepump in a torque proof manner. Usually, the balance drum is arranged atthe discharge side of the pump between the last stage impeller and ashaft sealing device. The balance drum defines a front side and a backside. The front side is the side facing the last stage impeller. Theback side is the side facing the shaft sealing device. A relief passageis disposed between the balance drum and a stationary part that isstationary with respect to the pump housing. The back side is usuallyconnected to the suction side of the pump by a balance line. Duringoperation there is a leakage flow through the relief passage from thefront side along the balance drum to the back side and from therethrough the balance line to the suction side. At the front side of thebalance drum the high pressure or the discharge pressure prevails, andat the back side essentially the suction pressure prevails. The pressuredifference between the front side and the back side results in a axialforce or an axial thrust which is directed in the opposite direction asthe axial thrust generated by the rotating impeller(s). Thus, the axialthrust that has to be carried by the axial or thrust bearing is at leastconsiderably reduced. Of course, the leakage flow along the balance drumresults in a decrease of the hydraulic performance or efficiency of thepump. Therefore, the relief passage is configured such, that the leakageflow is as low as possible but still sufficient for generating the axialthrust counteracting the axial thrust generated by the impeller(s).

SUMMARY

A centrifugal pump generally has at least one shaft seal device forsealing the shaft against a leakage of the fluid along the shaft. In aso-called between-bearing design the rotating shaft and all impellersare arranged between two shaft sealing devices, which are typicallyarranged next to the bearings at the drive end and at the non-drive endof the shaft, respectively.

The sealing devices can be configured for example as a mechanical seal.Typically, a mechanical seal comprises a stator and a rotor. The rotoris connected in a torque-proof manner with the shaft of the pump and thestator is fixed with respect to the pump housing such that the stator issecured against rotation. During rotation of the shaft the rotor is insliding contact with the stator thus performing the sealing action. Aliquid, e.g. the fluid conveyed by the pump or any other lubricant issupplied to the mechanical seal for generating a fluid film between thestator and the rotor.

A sealing device such as a mechanical seal requires cooling for removingthe heat from the sealing device, as well as flushing to keep particlesaway from the sealing elements. Therefore a certain flow is required forcooling and flushing. It is a known measure that the flow required toflush and to cool the sealing devices is extracted either at or near theoutlet of the pump or at an intermediate stage of the pump. Thisrequired flow for flushing the sealing devices causes additional losseswhich reduces the efficiency of the pump.

Nowadays many applications strived for the most efficient use of thepump. It is desirable to have the highest possible ratio of the power,especially the hydraulic power, delivered by the pump to the powerneeded for driving the pump. This desire is mainly based upon anincreased awareness of environment protection and a responsible dealingwith the available resources as well as on the increasing costs ofenergy.

It is therefore an object of the invention to propose a centrifugal pumpfor conveying a fluid, having a high efficiency without a reduction inthe operating safety of the pump.

The subject matter of the invention satisfying these objects ischaracterized by the features described herein.

Thus, according to an embodiment of the invention, a centrifugal pumpfor conveying a fluid is proposed, comprising a pump housing with aninlet at a suction side and an outlet at a discharge side, at least oneimpeller for conveying the fluid from the inlet to the outlet, a shaftfor rotating the impeller about an axial direction, a first sealingdevice for sealing the shaft at the suction side, a second sealingdevice for sealing the shaft at the discharge side, a balance drumfixedly connected to the shaft and arranged between the at least oneimpeller and the second sealing device, wherein the balance drum definesa front side facing the at least one impeller and a back side facing thesecond sealing device, wherein a relief passage is disposed between thebalance drum and a stationary part configured to be stationary withrespect to the pump housing, wherein the relief passage extends from thefront side to the back side, wherein a balance line is providedconnecting the back side with the suction side, wherein a dischargeopening is arranged at the relief passage between the front side and theback side, and wherein a connecting line is provided for connecting thedischarge opening with the first sealing device.

Thus, a part of the flow passing through the relief passage along thebalance drum is guided away from the relief passage through theconnecting line to the first sealing device and used for flushing andcooling the first sealing device. Therefore, there is no need to extractan additional flow of the fluid e.g. at the discharge side or at anintermediate stage of the pump. This results in an increase of theefficiency of the pump, because only the unavoidable leakage flowthrough the relief passage is used for flushing the first sealingdevice. There is no need for an additional take-off of pressurized fluidin order to flush the first sealing device.

At the first sealing device, or in the first seal housing/chamber,respectively, a pressure prevails that is at most slightly higher thanthe suction pressure at the suction side of the pump. The pressure atthe discharge opening in the relief passage is considerably higher thanthe suction pressure. Therefore the flow of fluid in the connecting lineis directed towards the first sealing device and can be used forflushing the first sealing device.

Preferably, the connecting line comprises at least one flow controlelement for controlling the flow through the connecting line. This hasthe advantage that the volumetric flow for flushing the first sealingdevice can be adjusted. The flow control element can be, for example, avalve or an orifice.

In order to make the pump even more efficient it is preferred that theconnecting line comprises a first branch and a second branch, whereinthe first branch is connected with the first sealing device, and thesecond branch is connected with the second sealing device. Thus, theflow discharged from the relief passage through the discharge openingand the connecting line is additionally used to also flush the secondsealing device.

According to a preferred configuration, the first branch comprises afirst flow control element for controlling the flow through the firstbranch, and the second branch comprises a second flow control elementfor controlling the flow through the second branch. By this measure boththe flow to the first sealing device and the flow to the second sealingdevice can be controlled.

Furthermore, it is preferred that the connecting line comprises a thirdbranch, wherein the third branch is connected to the suction side. Thethird branch can be connected for example to the inlet of the pump or tothe balance line or to a suction tank being in fluid communication withthe inlet of the pump. By the third branch, the flow extracted from therelief passage can be routed directly back to the suction side, i.e.without passing through one of the sealing devices, for example if theextracted flow exceeds the required flow for the sealing devices or ifthe pressure requires an adjustment. The third branch is particularlyadvantageous to adjust the leakage flow through the relief passage.

Preferably, the third branch comprises a third flow control element forcontrolling the flow through the third branch.

According to a preferred design at least one of the flow controlelements is configured as an adjustable valve.

For many embodiments it is advantageous that each flow control elementis configured as an adjustable valve.

Preferably, the first sealing device comprises a mechanical seal.

It is also preferred that the second sealing device comprises amechanical seal.

According to a preferred embodiment the pump is configured as amultistage pump having a plurality of impellers, wherein the impellersare arranged one after another on the shaft.

Furthermore, it is preferred that the pump is configured as abetween-bearing pump.

In particular, the pump can be configured as a barrel type pumpcomprising an outer barrel casing, in which the pump housing isarranged.

Further advantageous measures and embodiments of the invention willbecome apparent from the dependent claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be explained in more detail hereinafter withreference to the drawings.

FIG. 1 is a schematic cross-sectional view of an embodiment of acentrifugal pump according to the invention, and

FIG. 2 is a cross-sectional view illustrating a configuration of thebalance drum and the connecting line.

DETAILED DESCRIPTION

FIG. 1 shows a schematic cross-sectional view of an embodiment of acentrifugal pump according to the invention, which is designated in itsentity with reference numeral 1. The pump 1 is designed as a centrifugalpump for conveying a fluid, for example a liquid such as water.

The centrifugal pump 1 comprises a pump housing 2 having an inlet 3 andan outlet 4 for the fluid to be conveyed. The inlet 3 is arranged at asuction side S, where a suction pressure prevails, and the outlet 4 isarranged at a discharge side D, where a discharge pressure prevails. Thesuction pressure is also referred to as low pressure, and the dischargepressure is also referred to as high pressure. The centrifugal pump 1further comprises at least one impeller 5, 51 for conveying the fluidfrom the inlet 3 to the outlet 4 as indicated by the dashed arrowswithout reference numerals, as well as a shaft 6 for rotating eachimpeller 5, 51 about an axial direction A. The axial direction A isdefined by the axis of the shaft 6. Each impeller 5, 51 is mounted tothe shaft 6 in a torque proof manner. The shaft 6 has a drive end 61,which can be connected to a drive unit (not shown) for driving therotation of the shaft 6 about the axial direction. The drive unit cancomprise, for example, an electric motor. The other end of the shaft 6is referred to as non-drive end 62.

In the following description reference is made by way of example to anembodiment, which is suited for many applications, namely that thecentrifugal pump 1 is configured as a multistage pump 1 having aplurality of impellers 5, 51, wherein the impellers 5, 51 are arrangedone after another on the shaft 6. The reference numeral 51 designatesthe last stage impeller 51, which is the impeller 51 closest to theoutlet 4. The last stage impeller 51 pressurizes the fluid to thedischarge pressure. The embodiment shown in FIG. 1 has nine stages,which has to be understood exemplary. The plurality of impellers 5, 51can be arranged in an in-line configuration as shown in FIG. 1 or in aback-to-back configuration.

The multistage centrifugal pump 1 shown in FIG. 1 is designed as ahorizontal pump, meaning that during operation the shaft 6 is extendinghorizontally, i.e. the axial direction A is perpendicular to thedirection of gravity. In particular, the centrifugal pump 1 shown inFIG. 1 can be designed as a horizontal barrel casing multistage pump 1,i.e. as a double-casing pump. The multistage pump 1 can be designed, forexample, as a pump 1 of the pump type BB5 according to API 610. Whenconfigured as a BB5 type pump, the centrifugal pump 1 comprises an outerbarrel casing 100, in which the pump housing 2 is arranged.

It has to be understood that the invention is not restricted to thistype of centrifugal pump 1. In other embodiments, the centrifugal pumpcan be configured without an outer barrel casing, for example as a BB4type pump, or as an axially split multistage pump, or as a single stagepump, or as a vertical pump, meaning that during operation the shaft 6is extending in the vertical direction, which is the direction ofgravity, or as any other type of centrifugal pump.

The centrifugal pump 1 comprises bearings on both sides of the pluralityof impellers 5, 51 (with respect to the axial direction A), i.e. thecentrifugal pump 1 is designed as a between-bearing pump. A first radialbearing 81, a second radial bearing 82 and an axial bearing 83 areprovided for supporting the shaft 6. The first radial bearing 81 isarranged adjacent to the drive end 61 of the shaft 6. The second radialbearing 82 is arranged adjacent or at the non-drive end 62 of the shaft6. The axial bearing 83 is arranged between the plurality of impellers5, 51 and the first radial bearing 81 adjacent to the first radialbearing 81. The bearings 81, 82, 83 are configured to support the shaft6 both in the axial direction A and in a radial direction, which is adirection perpendicular to the axial direction A. The radial bearings 81and 82 are supporting the shaft 6 with respect to the radial direction,and the axial bearing 83 is supporting the shaft 6 with respect to theaxial direction A. The first radial bearing 81 and the axial bearing 83are arranged such that the first radial bearing 81 is closer to thedrive end 61 of the shaft 6. Of course, it is also possible to exchangethe position of the first radial bearing 81 and the axial bearing 83,i.e. to arrange the first radial bearing 81 between the axial bearing 83and the plurality of impellers 5, 51, so that the axial bearing 83 iscloser to the drive end 61 of the shaft 6.

A radial bearing, such as the first or the second radial bearing 81 or82 is also referred to as a “journal bearing” and an axial bearing, suchas the axial bearing 83, is also referred to as an “thrust bearing”. Thefirst radial bearing 81 and the axial bearing 83 can be configured asseparate bearings as shown in FIG. 1, but it is also possible that thefirst radial bearing 81 and the axial bearing 83 are configured as asingle combined radial and axial bearing supporting the shaft both inradial and in axial direction.

The second radial bearing 82 supports the shaft 6 in the radialdirection. In the embodiment shown in FIG. 1, there is no axial bearingprovided at the non-drive end 62 of the pump shaft 6. Of course, inother embodiments it is also possible that an axial bearing for theshaft 6 is provided at the non-drive end 62. In embodiments, where anaxial bearing is provided at the non-drive end 62, a second axialbearing can be provided at the drive end 61 or the drive end 61 can beconfigured without an axial bearing.

The centrifugal pump 1 further comprises two sealing devices, namely afirst sealing device 91 for sealing the shaft 6 at the suction side Sand a second sealing device 92 for sealing the shaft 6 at the dischargeside D. With respect to the axial direction A the first sealing device91 is arranged between the plurality of impellers 5 an the second radialbearing 82, and the second sealing device 92 is arranged between thelast stage impeller 51 and the axial pump bearing 83. Both sealingdevices 91, 92 seal the shaft 6 against a leakage of the fluid along theshaft 6 e.g. into the environment. Furthermore, by the sealing devices91 and 92 the fluid can be prevented from entering the bearings 81, 82,83. Preferably each sealing device 91, 92 comprises a mechanical seal.Mechanical seals are well-known in the art in many different embodimentsand therefore require no detailed explanation. In principle, amechanical seal is a seal for a rotating shaft 6 and comprises a rotorfixed to the shaft 6 and rotating with the shaft 6, as well as astationary stator fixed with respect to the pump housing 2. Duringoperation the rotor and the stator are sliding along each other—usuallywith a liquid as lubricant there between—for providing a sealing actionto prevent the fluid from escaping to the environment or entering thebearings 81, 82, 83. In many embodiments a separate bearing isolator isprovided which prevents liquids or solids to enter the bearings 81, 82,83. In such embodiments where separate bearing isolators are provided,the sealing devices 91, 92, e.g. the mechanical seals prevent the fluidfrom leaking into the environment.

The centrifugal pump 1 further comprises a balance drum 7 for at leastpartially balancing the axial thrust that is generated by the impellers5, 51 during operation of the centrifugal pump 1. The balance drum 7 isfixedly connected to the shaft 6 in a torque proof manner. The balancedrum 7 is arranged at the discharge side D between the last stageimpeller 51 and the second sealing device 92. The balance drum 7 definesa front side 71 and a back side 72. The front side 71 is the side facingthe last stage impeller 51. The back side 72 is the side facing thesecond sealing device 92. The balance drum 7 is surrounded by astationary part 21, so that a relief passage 73 is formed between theradially outer surface of the balance drum 7 and the stationary part 21.The stationary part 21 is configured to be stationary with respect tothe pump housing 2. The relief passage 73 forms an annular gap betweenthe outer surface of the balance drum 7 and the stationary part 21 andextends from the front side 71 to the back side 72. The front side 71 isin fluid communication with the outlet 4, so that the axial surface ofthe balance drum 7 facing the front side 71 is exposed essentially tothe discharge pressure prevailing at the outlet 4 during operation ofthe pump 1. Of course, due to smaller pressure losses caused by thefluid communication between the outlet 4 and the balance drum 7 thepressure prevailing at the axial surface of the balance drum 7 facingthe front side 71 can be somewhat smaller than the discharge pressure.However, the considerably larger pressure drop takes place over thebalance drum 7. At the back side 72 a chamber 74 is provided, which isconnected by a balance line 10 with the suction side S, e.g. with theinlet 3. The pressure in the chamber 74 at the back side 72 is somewhatlarger than the suction pressure due to the pressure drop over thebalance line 10 but considerably smaller than the discharge pressure.

Since the front side 71 is exposed essentially to the discharge pressureat the outlet 4 a pressure drop exists over the balance drum 7 resultingin a force that is directed to the right side according to therepresentation in FIG. 1 and therewith counteracts the axial thrustgenerated by the impellers 5, 51 during operation of the pump 1.

The balance line 10 recirculates the fluid from the chamber 74 at theback side 72 to the suction side S. A part of the pressurized fluidpasses from the front side 71 through the relief passage 73 to the backside 72, enters the balance line 10 and is recirculated to the suctionside S of the centrifugal pump 1. The balance line 10 constitutes a flowconnection between the back side 72 and the suction side S at the pumpinlet 3. The balance line 10 can be arranged—as shown in FIG. 1—outsidethe pump housing 2 and inside the barrel casing 100. In otherembodiments the balance line 10 can be designed as internal linecompletely extending within the pump housing 2. In still otherembodiments the balance line can be arranged outside the barrel casing100.

According to the invention, a discharge opening 70 is arranged at therelief passage 73 between the front side 71 and the back side 72 and aconnecting line 40 connects the discharge opening 70 with the firstsealing device 91. Thus, a part of the flow passing through the reliefpassage 73 enters the connecting line 40 through the discharge opening70 and is guided to the first sealing device 91 for flushing and coolingthe first sealing device 91. Due to the location of the dischargeopening 70 between the front side 71 and the back side 72 the pressureat the discharge opening 70 is an intermediate pressure, which issmaller than the discharge pressure at the outlet 4 of the pump 1 andlarger than the pressure in the chamber 74 at the back side 72 that is abit larger than the suction pressure at the suction side S of thecentrifugal pump 1. The pressure in the first sealing device 91, e.g.the pressure in the sealing chamber of the mechanical seal, is at mostslightly higher than the suction pressure, so that this pressure in thefirst sealing device 91 is considerably lower than the intermediatepressure prevailing at the discharge opening 70. Thus, the flowdischarged through the connecting line 40 can be used for flushing thefirst sealing device 91 in order to cool the first sealing device 91down and to keep particles away from the sealing elements of the firstsealing device 91. During operation of the centrifugal pump 1 a volumeof the pumped fluid is constantly extracted from the relief passage 72,guided through the connecting line 40 and injected into the firstsealing device 91 for flushing. Consequently, there is no need toextract pressurized fluid at any other location e.g. from the outlet 4or at an intermediate stage of the pump 1 for flushing the first sealingdevice 91. Only a part of the unavoidable leakage flow through therelief passage 73 along the balance drum 7 is used for flushing thefirst sealing device 91. Therefore, the efficiency of the centrifugalpump 1 is enhanced.

Referring now to FIG. 2 some preferred measures and variants areexplained, each of which can be realized in particular in the embodimentshown in FIG. 1. Since it is sufficient for the understanding, in FIG. 2only one impeller is shown, which can be for example the only impellerof a single stage pump or the last stage impeller 51 of a multistagepump.

FIG. 2 shows a cross-sectional view illustrating a configuration of thebalance drum 7 and the connecting line 40. The connecting line 40 aswell as the balance line 10 are at least partially represented as singlelines in FIG. 2, wherein the direction of flow through the particularline is indicated by the arrows without reference numeral. The Fluidflowing through the pump 1 is indicated by the dashed arrows withoutreference numeral.

Preferably, the connecting line 40 comprises at least one flow controlelement, namely a first flow control element (flow controller) 45, forcontrolling the flow through the connecting line 40 into the firstsealing device 91. The first flow control element 45 can be designed asa throttle or as a orifice or as a valve such as a flow control valve orany other adjustable valve. With the first flow control element 45 theflushing volumetric flow injected into the first sealing device 91 canbe adjusted.

As a further advantageous measure the connecting line 40 can comprise afirst branch 41 and a second branch 42, wherein the first branch 41 isconnected with the first sealing device 91, and the second branch 42 isconnected with the second sealing device 92. If the first flow controlelement 45 is provided in this design, the first flow control element 45is arranged in the first branch 41.

Flushing both the first sealing device 91 and the second sealing device92 with the flow extracted from the relief passage 73 through thedischarge opening 70 still increases the efficiency of the centrifugalpump 1, because there is no need to extract the flow for flushing thesecond sealing device 92 at any other location of the centrifugal pumpthan at the discharge opening 70 in the relief passage 73. Since thesecond sealing device 92 faces the chamber 74 at the back side 72 of thebalance drum 7, the pressure at or in the second sealing device 92 is atmost as high as the pressure at the back side 72, i.e. only slightlyhigher than the suction pressure. Thus, the pressure in the secondsealing device 92, e.g. the pressure in the sealing chamber of themechanical seal of the second sealing device 92, is considerably lowerthan the intermediate pressure at the discharge opening 70. Thus, theflow taken from the discharge opening 70 and guided through theconnecting line 40 can be injected into the second sealing device 92.

Preferably, the second branch 42 of the connecting line 40 comprises asecond flow control element (flow controller) 46, for controlling theflow through the second branch 42 into the second sealing device 92. Thesecond flow control element 46 can be designed as a throttle or as aorifice or as a valve such as a flow control valve or any otheradjustable valve. With the second flow control element 46 the flushingvolumetric flow injected into the second sealing device 92 can beadjusted.

It is a further preferred measure that the connecting line 40 comprisesa third branch 43, wherein the third branch 43 is connected to thesuction side S. Thus, a part of the flow discharged from the reliefpassage 73 through the discharge opening 70 can be directly recirculatedto the suction side S without passing through any of the sealing devices91, 92. The third branch 43 can be connected for example to the inlet 3of the centrifugal pump 1 or to a tank, from which the fluid is suppliedto the inlet 3 of the centrifugal pump 1. Furthermore, it is alsopossible, that the third branch 43 leads into the balance line 10.

Optionally, the third branch 43 of the connecting line 40 comprises athird flow control element (flow controller) 47, for controlling theflow through the third branch 43 leading to the suction side S. Thethird flow control element 47 can be designed as a throttle or as aorifice or as a valve such as a flow control valve or any otheradjustable valve.

It has to be noted that the preferred measures, in particular thoseexplained referring to FIG. 2, do not have to be realized all together.Each of the measures can be realized independently from the othermeasures. In addition, all combinations of specific measures can berealized.

The centrifugal pump 1 renders possible to control and to adjust thebalancing flow passing through the relief passage 72 and the balanceline 10, i.e. the flow that is recirculated through the balance line 10can be adjusted. Said adjustment can be realized by regulating the flowpassing through the discharge opening 70 into the connecting line 40.Thus, by controlling the flow through the connecting line 40 the balanceflow recirculated to the suction side S can be adjusted. This is inparticular advantageous for such embodiments of the centrifugal pump 1,that are designed for high to very high discharge pressures and a lowdischarge flow.

What is claimed:
 1. A centrifugal pump for conveying a fluid,comprising: a pump housing with an inlet at a suction side and an outletat a discharge side; at least one impeller configured to convey thefluid from the inlet to the outlet; a shaft configured to rotate theimpeller about an axial direction; a first sealing device configured toseal the shaft at the suction side; a second sealing device configuredto seal the shaft at the discharge side; a balance drum fixedlyconnected to the shaft and arranged between the at least one impellerand the second sealing device, the balance drum defining a front sidefacing the at least one impeller and a back side facing the secondsealing device; a relief passage disposed between the balance drum and astationary part configured to be stationary with respect to the pumphousing, the relief passage extending from the front side to the backside; a balance line connecting the back side with the suction side; adischarge opening arranged at the relief passage between the front sideand the back side; and a connecting line connecting the dischargeopening with the first sealing device.
 2. The centrifugal pump inaccordance with claim 1, wherein the connecting line comprises at leastone flow controller configured to control the flow through theconnecting line.
 3. The centrifugal pump in accordance with claim 1,wherein the connecting line comprises a first branch and a secondbranch, the first branch connected with the first sealing device, andthe second branch connected with the second sealing device.
 4. Thecentrifugal pump in accordance with claim 3, wherein the first branchcomprises a first flow controller configured to control the flow throughthe first branch, and the second branch comprises a second flowcontroller configured to control the flow through the second branch. 5.The centrifugal pump in accordance with claim 3, wherein the connectingline comprises a third branch, the third branch is connected to thesuction side.
 6. The centrifugal pump in accordance with claim 5,wherein the third branch comprises a third flow controller configured tocontrol the flow through the third branch.
 7. The centrifugal pump inaccordance with claim 6, wherein at least one of the flow controllers isan adjustable valve.
 8. The centrifugal pump in accordance with claim 2,wherein the at least one flow controller includes a plurality of flowcontrollers and each flow controller is an adjustable valve.
 9. Thecentrifugal pump in accordance with claim 1, wherein the first sealingdevice comprises a mechanical seal.
 10. The centrifugal pump inaccordance with claim 1, wherein the second sealing device comprises amechanical seal.
 11. The centrifugal pump in accordance with claim 1,wherein the pump is a multistage pump and the at least one impellerincludes a plurality of impellers, and the impellers are arranged oneafter another on the shaft.
 12. The centrifugal pump in accordance withclaim 11, wherein the pump is a between-bearing pump.
 13. Thecentrifugal pump in accordance with claim 11, further comprising anouter barrel casing, in which the pump housing is arranged.